1. Field of the Invention
The present invention relates to a circuit for accurately converting digitized analog signals into digital signals, and more particularly to a circuit for converting digitized analog video signals into digital video signals served to a liquid crystal display (hereinafter referred as LCD) panel.
2. Description of the Prior Art
Owing to the increasing functions of a personal computer, analog signals such as video signals or audio signals can be processed and modified by the multimedia unit of a computer, the processed digital signals must be reverted to analog signals by a D/A converter such that the signals can be received by a common speaker or a conventional CRT monitor. Such reverted analog signals are different from original analog signals, which as shown in FIG. 1(a), are referred as "digitized analog signals", as shown in FIG. 1.
If the digitized analog signals must be sampled again by some digital devices (devices that receive digital signals only, e.g. LCD panels) to be converted to digital signals, some conditions must be satisfied for the digitized analog signals to be accurately reverted to the original digital signals. Next, the video signals and the conditions that are applied to an LCD panel are used to exemplify the sampling process.
An LCD panel is a device which can only receive digital signals. However, video signals output from a display card of a common personal computer are digitized analog signals as stated in the above. When such video signals enter an LCD panel, an A/D converter must be used to perform sampling such that digital signals are obtained. If the sampling cannot be performed accurately or the sampling is performed ambiguously, the displayed picture will become distorted or unclear.
Referring to FIG. 2, in which the sampling clock CLK (triggering at rising edges) is not a proper sampling signal, for not only does it cause the digital signal to become distorted (comparing the digital signal b.sub.0 b.sub.1 b.sub.2 b.sub.3 b.sub.4 b.sub.5 shown in the lower FIG. 2 with the digitized analog signal shown in the upper FIG. 2), but also produce some imprecise signals.
FIG. 3 shows three different kinds of sampling clocks CLK1, CLK2 and CLK3, in which only CLK3 is an ideal sampling clock. This is illustrated in the following: the actuating points of CLK1 are not ideal, because ambiguous results are produced repetitively. Accordingly, the ambiguous results that are produced repetitively can be prevented only when the frequency of the sampling clock is the same as that of the original digital signal of the digitized analog signal. Although CLK2 satisfies the above conditions, but the actuating points of most digital electrical elements have limitations on setup time and hold time, and the requirements on setup time and hold time are different for different elements. In FIG. 3, the sampling points of CLK2 are at the changing edges of the signal. Thus, for an A/D converter, the sampled data may be signals that are before or after the change. Therefore the actuating points of the sampling are not ideal.
CLK3 is an ideal sampling clock, because a precise signal can be sampled by each of the actuating points of the sampling, and no signal is omitted. Thus it is a good sampling clock.